Bulletin 90-02: Loss of Thermal Margin Caused by Channel Box Bow
OMB No.: 3150-0011
NRCB No. 90-02
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 20, 1990
NRC BULLETIN NO. 90-02: LOSS OF THERMAL MARGIN CAUSED BY CHANNEL BOX BOW
All holders of operating licenses (OLs) or construction permits (CPs) for
boiling water reactors (BWRs).
The purpose of this bulletin is to request that addressees determine whether
any channel boxes are being reused after their first bundle lifetime and, if
so, ensure that the effects of channel box bow on the critical power ratio
(CPR) calculation are properly taken into account.
As a result of information obtained at a meeting on fuel failures caused by
dryout at a foreign BWR facility and at meetings with BWR fuel vendors, the
NRC issued the attached Information Notice 89-69, "Loss of Thermal Margin
Caused by Channel Box Bow," on September 29, 1989, to alert addressees to
the potential problems associated with excessive channel box bow that could
result in a loss of thermal margin. The main concern applicable to the U.S.
BWRs at this time is that the channel box bow effect has not been taken into
account in the CPR calculation for channel boxes in their second bundle
Because channel box bow has a non-negligible impact on BWR thermal limits
calculations, it is important that actions are taken to prevent the
possibility of any fuel failures because of this effect. The staff
acknowledges that the possibility of fuel failure as a result of channel box
bow for channel boxes in the first fuel bundle lifetime is remote for U.S.
BWRs. However, channel boxes in use for a second bundle lifetime may be
severely bowed with a loss of CPR margin of up to 0.20 delta-CPR.
Therefore, the staff has concluded that all addressees that currently use
channel boxes for a second bundle lifetime should inform the NRC of the
number and disposition of such channel boxes in the core, and describe the
actions taken to assure compliance with the technical specification CPR
limits. Because of the new information about channel box bow, the staff
expects that reuse of channel boxes in future core reloads will be addressed
as an unreviewed safety question in reload core safety evaluations.
March 20, 1990
Page 2 of 3
Addressees who use channel boxes for only a single bundle lifetime are
expected to take into account the effects of channel box bow in analysis
supporting the next fuel reload application. This can be done by either the
use of an approved methodology that takes the effects of channel box bow
into account in the CPR calculations, or by the use of a bounding value of
delta-CPR. Preliminary results have been obtained by both General Electric
Company (GE) and Advanced Nuclear Fuels Corporation (ANF) in their proposed
methodologies. It is expected that the delta-CPR caused by channel box bow
would not exceed 0.04 for D-lattice plants and would not exceed 0.02 for
C-lattice and S-lattice plants. These results are based on the maximum
expected channel box bow for channel boxes that are used for only a single
bundle lifetime. Because of the increased rate of channel growth observed
at the foreign BWR facility after about 50 to 60 GWd/MTU burnup, the
delta-CPR effect is expected to be much larger for channel boxes being used
in their second bundle lifetime. However, GE and ANF have not provided
recommendations for the use of second bundle lifetime channel boxes.
Both GE and ANF have provided the NRC with reports describing their
methodology for incorporating the effects of channel box bow in their CPR
analysis. The GE report was submitted to the NRC by letter dated November
15, 1989, and is entitled "Effect of Channel Bow on Margins to Core Thermal
Limits in BWRs." The ANF report was submitted to the NRC by letter dated
November 30, 1989, and is entitled "Critical Power Methodology for Boiling
Water Reactors - Methodology for Analysis of Assembly Channel Bowing
Effects," ANF-524(P), Revision 2, Sup-plement 1. These two methodologies are
presently under staff review, which is expected to be completed by May 1990.
In the interim, the staff will consider approval of proposed plant- and
cycle-specific applications of these new methodologies, if it can be
demonstrated that the new methods are conservative in comparison to
previously approved methods and are being correctly implemented.
In summary, all BWR licensees are expected, in evaluation of the design and
operation of reload cores, either to implement an approved methodology or to
implement an acceptable bounding value of delta-CPR, as needed, to assure
that the effects of channel box bow on CPR limits are taken into account to
assure compliance with technical specification CPR limits.
All BWR licensees that currently use channel boxes for a second bundle
lifetime should verify that current CPR technical specification operating
and safety limits are met.
Within 30 days of receipt of this bulletin, all holders of OLs for BWRs that
currently use channel boxes for a second bundle lifetime shall advise the
NRC of the number of such channel boxes and their disposition in the core.
In addition, the response shall describe the methods, and the associated
data base, used to account for the effects of channel box bow during the
second bundle lifetime use of channel boxes to ensure conformance with the
CPR technical specification operating and safety limits.
March 20, 1990
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The written reports required above shall be submitted under oath or
affirmation pursuant to Section 182 of the Atomic Energy Act of 1954, as
amended (42 U.S.C. 2232), and shall be addressed to the U.S. Nuclear
Regulatory Commission, ATTN: Document Control Desk, Washington, D.C. 20555.
In addition, a copy shall be submitted to the appropriate Regional
The action requested in this bulletin ensures licensees' compliance with
their technical specifications as required under 10 CFR 50.36(c)(1)(i)(A).
The safety limit CPR comes under this regulation. A known deficiency in
current calculation of the CPR could result in fuel failures because the
operators are misled to believe they are in compliance with the technical
specification operating CPR limit. The requested action was evaluated
consistent with the provisions of 10 CFR 50.109 and found to be covered by
the provisions of paragraph (a)(4)(i).
This request is covered by Office of Management and Budget Clearance Number
3150-0011 which expires January 31, 1991. The estimated average burden
hours are 100 person-hours per licensee response, including assessment of
the new recommendation, searching data sources, gathering and analyzing the
data, and preparing the required letters. These estimated average burden
hours pertain only to these identified response-related matters. Send
comments regarding this burden estimate or any other aspect of this
collection of information, including suggestions for reducing this burden,
to the Information and Records Management Branch, Division of Information
Support Services, Office of Information Resources Management, U.S. Nuclear
Regulatory Commission, Washington, D.C. 20555; and to the Paperwork
Reduction Project (3150-0011), Office of Management and Budget, Washington,
If you have any questions about this matter, please contact one of the
technical contacts listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Peter C. Wen, NRR
Daniel B. Fieno, NRR
1. Information Notice 89-69, "Loss of Thermal Margin Caused by Channel
2. List of Recently Issued NRC Bulletins
Page Last Reviewed/Updated Friday, May 22, 2015